Image.Load(byte[]) should return non-generic Image

[antonfirsov]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

Discovered an API bug while answering #1571. The fix is a breaking change, but very scoped one.

[codecov]

Codecov Report

Merging #1573 (583bb6b) into master (36018bd) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1573   +/-   ##
=======================================
  Coverage   83.65%   83.65%           
=======================================
  Files         746      746           
  Lines       32991    32991           
  Branches     3680     3680           
=======================================
  Hits        27597    27597           
  Misses       4680     4680           
  Partials      714      714           

Flag	Coverage Δ
unittests	83.65% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/ImageSharp/Image.FromBytes.cs	93.90% <100.00%> (ø)

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[JimBobSquarePants]

Good find!